Imaging techniques such as computed tomography (CT) are often used to aid in directing the insertion of one or more probes into the patient's body during percutaneous biopsies and other similar procedures (e.g., hyperthermia probes, radiation seeds, etc.). In CT aided biopsies, for example, a scan is typically obtained to visualize the lesion to be biopsied. Using the image generated during the scan, the target site and the entry point on the patient's skin are selected and marked on the cathode ray tube (CRT) screen. The computer then calculates the distance and the angle of the insertion in order to reach the target site and displays the biopsy path on the screen.
The entry point is then marked on the patient's skin and the needle or probe is advanced into the patient's skin and tissues under free-hand control by the physician. Because of anatomy and overlying structures, the path selected for entry of the needle or probe is often not vertical but rather at an angle to the vertical. Often times a compound angle--that is, an angle with respect to the vertical in both the sagittal plane (i.e., the plane extending from head to toe) and the axial plane (i.e., the plane extending across the body from one side to the other) --must be used in order to avoid penetrating areas such as vital organs.
Although such procedures may be performed using a free-hand technique, it is difficult to ensure sufficient accuracy in orienting and maintaining the needle at the correct angle. The free-hand technique is even less precise and even more difficult when a compound angle is used. For example, in deep biopsies, even a few degrees deviation in angulation will result in a significant error in needle position. This can have undesirable consequences when the target lesion (e.g., tumor) is small and the path is close to surrounding vital structures, such as the aorta and nerves.
One solution is to direct a light or laser beam toward the target so that the physician can guide the insertion of the needle or probe along the line established by the beam. One such guidance apparatus is disclosed in the inventors' U.S. application Ser. No. 08/356,150, filed Dec. 15, 1994, entitled Apparatus for Performing Biopsies and the Like, which is hereby incorporated by reference in its entirety. In such apparatus, rotary stages are used to set the light beam angle in two orthogonal planes. For optimal use, such an apparatus requires the physician to level the structure supporting the rotary stages so that a setting of 0.degree. on the rotary stages corresponds to the vertical direction.
It would be desirable to provide a leveling device that is especially adapted for use in an apparatus for performing light beam guided biopsies or other techniques in which it was necessary to level a portion of the structure.